The current tendency is toward fabricating slim, compact, lightweight and high-performance electronic products, including microphones. A microphone is used to receive sound and convert acoustic signals into electric signals. Microphones are extensively used in daily-life appliances, such as telephones, mobiles phones, recording pens, etc. For a capacitive microphone, variation of sound forces the diaphragm to deform correspondingly in a type of acoustic waves. The deformation of the diaphragm induces capacitance variation. The variation of sounds can thus be obtained via detecting the voltage difference caused by capacitance variation.
Distinct from the conventional electret condenser microphones (ECM), mechanical and electronic elements of MEMS (Micro Electro-Mechanical Systems) microphones can be integrated on a semiconductor material by the IC (Integrated Circuit) technology to fabricate a miniaturized microphone. Now, MEMS microphones have become the mainstream of miniaturized microphones. MEMS microphones have advantages of compactness, lightweightness and low power consumption. Further, MEMS microphones can be fabricated with a surface-mount method, can bear a higher reflow temperature, can be easily integrated with a CMOS process and other audio electronic devices, and are more likely to resist radio frequency (RF) and electromagnetic interference (EMI).
Refer to FIG. 1 for a diagram schematically showing the structure of a conventional MEMS microphone. The conventional MEMS microphone 1 comprises a back plate 2, a diaphragm 3 and a spacer 4. The spacer 4 is interposed between the back plate 2 and the diaphragm 3 to insulate the diaphragm 3 from the back plate 2 and make the back plate 2 and the diaphragm 3 parallel to each other. Thus, the back plate 2 and the diaphragm 3 respectively form a lower electrode and an upper electrode of a parallel capacitor plate. The back plate 2 has a plurality of air holes 5 which are corresponding to the diaphragm 3 penetrating the back plate 2. The air holes 5 intercommunicate with a back chamber 7 formed on a silicon substrate 6.
Applying voltage to the back plate 2 and diaphragm 3 makes them respectively carry opposite charges and form a capacitor structure. A capacitance equation correlates to a parallel electrode plate is C=εA/d, wherein ε is the dielectric constant, A is the overlapped area of the two electrode plates, and d is the gap between the two capacitor plates. According to the equation, variation of the gap between the two capacitor plates will change the capacitance. When an acoustic wave causes the diaphragm 3 to vibrate and deform, the gap between the back plate 2 and the diaphragm 3 varies. Thus, the capacitance also varies to be converted into electric signals and output. The disturbed or compressed air between the diaphragm 3 and the back plate 2 is released to the back chamber 7 via the air holes 5 lest drastic pressure damage the diaphragm 3 and the back plate 2.
Refer to FIG. 2 for a diagram schematically showing the package structure of a conventional MEMS microphone. The conventional MEMS microphone 1 is installed on a baseplate 8 and packaged inside a holding space formed by a metallic cover 9. The diaphragm 3 and the back plate 2 are respectively electrically connected with a conversion chip 10. The conversion chip 10 converts the variation of the capacitance between the back plate 2 and the diaphragm 3 into electric signals to be output.
The conventional MEMS microphones adopt a flexible diaphragm. The sound pressure induces the deformation of the diaphragm and changes the gap between the diaphragm and the back plate, whereby the capacitance is varied. However, the flexible diaphragm is fabricated with a film-deposition method at a very high temperature. As different materials respectively have different thermal expansion coefficients, the diaphragm would accumulate tensile or compressive stress with different levels. Residual stress on the diaphragm will cause the warping or buckles of the diaphragm and lower the precision of detection. Moreover, due to the sensitivity of a microphone is inversely proportional to the residual stress of the diaphragm, higher residual stress results in low sensitivity. An U.S. Pat. No. 5,490,220 entitled “Solid State Condenser and Microphone Devices” proposes a suspended diaphragm without the constant boundary, wherein a cantilever is used to support the diaphragm, such that the diaphragm is suspended to release stress caused by temperature effect. Another U.S. Pat. No. 5,870,482 entitled “Miniature Silicon Condenser Microphone” designs a large plate diaphragm with only one side fastened.
However, a flexible diaphragm cannot be always parallel to the back plate when deforming. Thus, it is hard to estimate variation of the gap between the diaphragm and the back plate, and the precision is insufficient. Moreover, the sensitivity of a microphone is proportional to the driving voltage. When a higher voltage is used to enhance the sensitivity of a microphone, the conventional flexible diaphragm may collapse and attach to the back plate. In such a case, the microphone fails.